Sugar beet (Beta vulgaris L.) leaves contain virus-inducible type 1 (single chain) ribosome-inactivating proteins that have been named beetins. The structural and functional characterization, the cellular location, and the potential role of beetins as antiviral agents are reported here. Beetins are formed of a single polypeptide chain with a varying degree of glycosylation and strongly inhibited in vitro protein synthesis in rabbit reticulocyte lysates (IC50=1.15 ng ml(-1)) and a Vicia sativa L. cell-free system (IC50=68 ng ml(-1)) through the single depurination of the large rRNA. Beetins trigger the multidepurination of tobacco mosaic virus (TMV) genomic RNA which underwent extensive degradation upon treatment with acid aniline. Beetins are extracellular proteins that were recovered from the apoplastic fluid. Induction of sugar beet RIPs with either H2O2 or artichoke mottled crinkle virus (AMCV) was observed in leaves distant from the site of application of such elicitors. The external application of purified beetin to sugar leaves prevented infection by AMCV which supports the preliminary hypothesis that beetins could be involved in plant systemic acquired resistance subjected to induction by phytopathogens.